With the rapid expansion of the wireless LAN market, the industry has developed more and more complex modulation technologies in order to increase the data processing rate for components used in the wireless LAN. Among them, the requirements for radio receivers has become more stringent in terms of noise and linearity performance. Since direct conversion has been the most attractive receiver architecture for wideband wireless receivers, the issue of reducing flicker noise is one of the interests of all designers. Moreover, integration of wireless systems also drives the demands for the prior-manufacture co-verification of baseband and radio receivers to ensure the robust performance of the receiver.
The performance of CMOS direct conversion receivers rely very much on the mixer design it uses, since it may induce the DC offset, the even-order distortion and the flicker noise. The flicker noise issue is the greatest design obstacle since CMOS devices are surface channel devices that have worse flicker noises in comparison with that of the bipolar devices. The down converted received signal of a direct conversion receiver is located at the baseband and is easily corrupted by the flicker noise. In addition, as the multi-carrier modulation becomes more popular in wireless communication applications, receivers are facing more strict requirements in their linearity performance and such requirements are obviously more important as both the die size and the power supply are continuously scaling down. Minimizing the flicker noise while achieving sufficient conversion gain and good linearity has thus becomes essential in the design of the CMOS active mixer for direct conversion receivers. The Gilbert-cell architecture is the conventional CMOS active mixer that shows significant design tradeoffs among conversion gain, noise and inter-modulation distortions.
FIG. 1 illustrates the circuit diagram of a conventional single-balanced Gilbert-cell mixer with injection current. As shown in this figure, the mixer circuit 100 is used to combine an RF signal and a differential input signal. The mixer circuit 100 includes a switching stage 110 and an input stage 120. The switching stage 110 receives a differential input signal LO+, LO− and produces a differential output signal IF+, IF−. The switching stage 110 has a pair of NMOS transistors M2 and M3, which sources are coupled to a common node X. The switching stage 110 further includes a pair of load resistors RL1, RL2, which connect the drains of the transistors M2, M3 and a supply voltage VDD, respectively. The input stage 120 has an NMOS transistor M1 that receives an input of radio frequency signal RF. An injection current Iinj provides bias to the input stage 120. The input stage 120 uses the RF input to produce an RF voltage. The RF voltage is coupled to the common node X. A modulated output is generated by the mixer circuit 100 by including the RF signal to the input stage 120 to modulate the bias currents in transistors M2 and M3, such that the gain of the switching stage 110 varies in proportion to the amplitude of the LO signal.
The flicker noise of a single-balanced Gilbert-cell mixer comes from both the input stage and the switching stage. It is believed that the switching stage 110 dominates the flicker noise contribution at the mixer output. To minimize the flicker noise generated from the switching stage, the dimension of the switching pair M2, M3 can be made larger but this increases the noise contribution of the in-direct mechanism of the flicker noise through the parasitic capacitance at the common node X of the sources coupled switching pair M2, M3.
In order to solve this problem, a proposal to reduce the flicker noise may be seen in Manstretta et al., “Low 1/f Noise CMOS Active Mixer for Direct Conversion”, IEEE Trans. Circuits and Systems-II: Analog and Digital Signal Processing, vol. 48, No. 9, pp. 846-850, September 2001, wherein the tail current of the switching stage and the current of the input stage are separated to decouple the tradeoff.
In addition, a solution was proposed to add a capacitor connecting the gate of the injection current Iinj and the RF input, in order to reuse the injection current as part of the input stage. The reuse of injection current improves the gain of the input stage and thus reduces the noise contributed by the switching stage. However, under such a design, the common node X is tied to the drains of the input stage, which leaks the input stage flicker noise to the output due to the mismatch of the switching pair M2, M3. Such leakage is not negligible, since in such an architecture flicker noise in the input stage 120 is even important with that of the switching stage 110.
In Phan et al., “A High Performance COMS Direct Down Conversion Mixer for UWB System”, GLSVLSI'04, Apr. 26-28, 2004, a double-balanced Gilbert-type mixer is proposed. In the proposal, an inductor is provided at between the input stage and the common node to decouple the flicker noise. However, such solution does not decouple the flicker noise from the input stage of the mixer.
It is thus necessary to provide a novel current-folded mixer wherein the flicker noise may be effectively reduced.
It is also necessary to provide a current-folded mixer with low flicker noise while higher conversion gain and linearity may be maintained.
It is also necessary to provide a simplified current-folded mixer with relative low flicker noise.